Controls on evapotranspiration in a west Siberian bog

<jats:p>This study analyzed controls on evapotranspiration (<jats:italic>E</jats:italic>) from a western Siberian bog, from a perspective that surface constraints limit the <jats:italic>E</jats:italic> available for atmospheric evapotranspiration demands. Ratios of <jats:italic>E</jats:italic> to potential evapotranspiration (<jats:italic>E</jats:italic><jats:sub><jats:italic>P</jats:italic></jats:sub>) ranged from 0.2 to 0.9, and were clearly related to changes in surface constraints represented by the bulk transfer coefficient for latent heat <jats:italic>C</jats:italic><jats:sub><jats:italic>E</jats:italic></jats:sub> (= β<jats:italic>C</jats:italic><jats:sub><jats:italic>H</jats:italic></jats:sub> where β is the surface moisture availability and <jats:italic>C</jats:italic><jats:sub><jats:italic>H</jats:italic></jats:sub> is the bulk transfer coefficient for sensible heat). Both <jats:italic>E</jats:italic><jats:sub><jats:italic>P</jats:italic></jats:sub> and equilibrium evaporation (<jats:italic>E</jats:italic><jats:sub><jats:italic>EQ</jats:italic></jats:sub>) showed similar seasonal trends, suggesting the importance of radiation to the seasonal variation of <jats:italic>E</jats:italic><jats:sub><jats:italic>P</jats:italic></jats:sub>. The atmospheric drying power (<jats:italic>E</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub>) was a minor factor in <jats:italic>E</jats:italic><jats:sub><jats:italic>P</jats:italic></jats:sub> and showed less seasonal change during most of the growing season. However, the presence of a dry air mass caused by synoptic scale advection (most frequently observed in May 1999) significantly enhanced <jats:italic>E</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub>; consequently, the seasonal maximum of <jats:italic>E</jats:italic><jats:sub><jats:italic>P</jats:italic></jats:sub> occurred earlier than the seasonal maximum of <jats:italic>E</jats:italic><jats:sub><jats:italic>EQ</jats:italic></jats:sub>. Values for <jats:italic>C</jats:italic><jats:sub><jats:italic>H</jats:italic></jats:sub> (0.004–0.011) increased with leaf area index except <jats:italic>Sphagnum</jats:italic> moss (LAI<jats:sub><jats:italic>g</jats:italic></jats:sub>), indicating that vegetation growth contributes to changes in bog roughness through canopy height changes. The β value gradually decreased with decreases in the water table position (<jats:italic>z</jats:italic><jats:sub><jats:italic>wt</jats:italic></jats:sub>); the open water surface area and water content of <jats:italic>Sphagnum</jats:italic> moss depended on <jats:italic>z</jats:italic><jats:sub><jats:italic>wt</jats:italic></jats:sub>. Furthermore, the absence of a significant relationship between β and phenology implies that changes in evaporation contribute to variations in <jats:italic>E</jats:italic> more than changes in transpiration. Hence roughness change created by vegetation growth and surface wetness limit evapotranspiration to less than the potential evapotranspiration.</jats:p>